Materials which have the ability to reversibly change shape on application of an energy source such as heat are known as "shape memory" materials. Certain metal alloys such as Cu--Al--Ni, Au--Cd, In--Ti, Ni--Ti, can exhibit this property and are therefore often used in applications such as temperature sensors. Uses for such shape memory metal alloys however has been limited due to the available temperature ranges of operation for these alloys, and the expense of the base metals.
As an alternative to metal alloys, a number of polymers are known to exhibit shape memory behavior. The key to such behavior is the morphology of the polymer above and below its glass transition temperature (Tg) and its ability to form a partially crystalline rubbery state between the fluid state and the glassy state. In this rubbery state, the polymer can quite easily be deformed into any new shape, and, when the polymer is cooled below its Tg, the deformation is fixed and the new shape is retained. At this stage the polymer lacks its rubbery elasticity and is rigid. However, the original shape can be recovered simply by heating the polymer once again to a temperature higher than the Tg. Thus it is crucial that the selection of a shape memory polymer used to prepare a fabricated article involves both a knowledge of the final modulus and stiffness required, as well as a comparison of the Tg of the polymer with the operating temperature range to be used in the articles fabrication, and the intended temperature range predicted for its application.
For instance, selection of shape memory polymers for use in toy applications will require the Tg of the polymer used to make the toy structure or fabricated article to be in a fairly narrow range slightly above room temperature, allowing for the application of safe and relatively gentle heating to transform the toy into its moldable state and new configuration. Similarly, simple cooling back to room temperature will result in the new configuration having the original stiffness and modulus. Recovery of the original toy shape or configuration will again require only the application of safe and relatively gentle heating.
To date there have been a number of disclosures of shape memory polymers and articles and applications therefrom. U.S. Pat. No. 5,189,110 describes a shape memory polymer resin composition which is an A-B-A block copolymer where block A is a homopolymer or copolymer of a vinyl aromatic compound (or a hydrogenated product thereof), and block B is a homopolymer or a copolymer of butadiene and/or a hydrogenated product thereof. U.S. Pat. No. 5,098,776 describes a shape memory fibrous sheet comprising a powder of shape memory polymers of urethane, styrene/butadiene, crystalline diene, or norbornane. U.S. Pat. No. 5,093,384 describes a heat insulator made of a shape memory polymer foam in which the polymer foam is a polyurethane containing approximately equal amounts of NCO and OH groups at the terminals of the molecular chains. U.S. Pat. No. 5,634,913 describes a softening conduit for carrying fluids into and out of the human body having a needle like structure in which the tip is formed from a shape memory polymer such as a polyurethane. U.S. Pat. No. 5,552,197 describes a dynamic polymer composite comprising a multitude of fibers within a polymer matrix which is made from a shape memory polymer. U.S. Pat. No. 5,049,591 describes an open or closed cell shape memory polymer foam prepared from polymers of urethane, styrenebutadiene, crystalline diene, or norbornane. U.S. Pat. No. 5,066,091 describes an amorphous memory polymer alignment device wherein the amorphous memory polymer constituent is a covalently cross-linked semi-crystalline polymer such as polyethylene or ethylvinyl acetate copolymers with the polymers of esters of methacrylic acid and aliphatic, or aromatic alcohols being particularly preferred. U.S. Pat. No. 5,445,140 describes an endoscopic surgical device in which the hinge member is manufactured from a shape memory polymer which is preferably a polyurethane. U.S. Pat. No. 5,192,301 describes a closing plug device having a flange or an enlarged end portion made of a shape memory polymer such as polynorbornene, styrene-butadiene copolymer, polyurethane, transpolyisoprene and the like.
Thus the majority of current shape memory polymers are derived from either urethane- or A-B-A block structure styrene/butadiene/styrene-polymers. Such polymers may additionally require a further cross-linking transformation to allow the polymer to exhibit the desired shape memory behavior, thus further restricting the choice of polymer for a given application. In addition, many polymers are required to have very high molecular weight to function in these applications (e.g. 2,000,000 or higher) which in turn severely limits their processability and hence their use in many forming processes. Finally, many applications for shape/memory polymers require a precise and often narrow operating temperature range and a specific or minimum modulus, whilst the current technologies offer only broad temperature operating windows.
Hence it would be desirable to have a shape memory polymer composition which does not require cross-linking, exhibits excellent processability, and which has the capacity for precisely tuning both its glass transition process (peak temperature, amplitude and width of transition) as well as the stiffness and modulus of the material in its final state.
There are a wide range of structures and fabricated articles which would benefit from being prepared from polymer compositions which can afford both Tg and modulus control and preferentially exhibit the shape memory property. Such structures and fabricated articles can include, but are not limited to, fibers, foams, films and molded materials.
Fibers are often classified in terms of their diameter which can be measured and reported in a variety of fashions. Generally, fiber diameter is measured in denier per filament. Denier is a textile term which is defined as the grams of the fiber per 9000 meters of that fiber's length. Monofilament generally refers to an extruded strand having a denier per filament greater than 15, usually greater than 30. Fine denier fiber generally refers to fiber having a denier of about 15 or less. Microdenier (aka microfiber) generally refers to fiber having a diameter not greater than about 100 micrometers. The fiber can also be classified by the process by which it is made, such as monofilament, continuous wound fine filament, staple or short cut fiber, spun bond, and melt blown fiber. Fiber can also be classified by the number of regions or domains in the fiber.
The shape memory fibers of the present invention include the various homofil fibers made from the substantially random interpolymers or blend compositions therefrom. Homofil fibers are those fibers which have a single region (domain) and do not have other distinct polymer regions (as do bicomponent fibers). These homofil fibers include staple fibers, spunbond fibers or melt blown fibers (using, e.g., systems as disclosed in U.S. Pat. No. 4,340,563 (Appel et al.), U.S. Pat. No. 4,663,220 (Wisneski et al.), U.S. Pat. No. 4,668,566 (Braun), or U.S. Pat. No. 4,322,027 (Reba), all of which are incorporated herein by reference), and gel spun fibers (e.g., the system disclosed in U.S. Pat. No. 4,413,110 (Kavesh et al.), incorporated herein by reference). Staple fibers can be melt spun (i.e., they can be extruded into the final fiber diameter directly without additional drawing), or they can be melt spun into a higher diameter and subsequently hot or cold drawn to the desired diameter using conventional fiber drawing techniques.
The novel shape memory fibers disclosed herein can also be used as bonding fibers, especially where the novel fibers have a lower melting point than the surrounding matrix fibers. In a bonding fiber application, the bonding fiber is typically blended with other matrix fibers and the entire structure is subjected to heat, where the bonding fiber melts and bonds the surrounding matrix fiber. Typical matrix fibers which benefit from use of the novel shape memory fibers of the present invention include, but are not limited to, synthetic fibers, such as fibers made from, poly(ethylene terephthalate), polyproylene, nylon, other heterogeneously branched polyethylenes, linear and substantially linear ethylene interpolymers and polyethylene homopolymers. Also included are the various natural fibers which include, but are not limited to, those made from silk, wool, and cotton. The diameter of the matrix fiber can vary depending upon the end use application.
The shape memory fibers of the present invention also include the various composite bicomponent fibers which can comprise the substantially random interpolymers and a second polymer component. This second polymer component can be an ethylene or .alpha.-olefin homopolymer or interpolymer; an ethylene/propylene rubber (EPM), ethylene/propylene diene monomer terpolymer (EPDM), isotactic polypropylene; a styrene/ethylene-butene copolymer, a styrene/ethylene-propylene copolymer, a styrene/ethylene-butene/styrene (SEBS) copolymer, a styrene/ethylene-propylene/styrene (SEPS) copolymer; the acrylonitrile-butadiene-styrene (ABS) polymers, styrene-acrylonitrile (SAN), high impact polystyrene, polyisoprene, polybutadiene, natural rubbers, ethylene/propylene rubbers, ethylene/propylene diene (EPDM) rubbers, styrene/butadiene rubbers, thermoplastic polyurethanes, epoxies, vinyl ester resins, polyurethanes, phenolic resins, homopolymers or copolymers of vinyl chloride or vinylidene chloride, poly(methylmethacrylate), polyester,nylon-6, nylon-6,6, poly(acetal); poly(amide), poly(arylate), poly(carbonate), poly(butylene) and polybutylene, polyethylene terephthalates; or blend compositions therefrom. Preferably the second polymer component is an ethylene or .alpha.-olefin homopolymer or interpolymer, wherein said u-olefin has from 3 to 20 carbon atoms, or polyethylene terephthalate.
The bicomponent fibers have the two polymers in a co-continuous phase. Examples of such bicomponent fiber configurations and shapes include sheath/core fibers in which the perimeter shape is round, oval, delta, trilobal, triangular, dog-boned, or flat or hollow configurations. Other types of bicomponent fibers within the scope of the invention include such structures as segmented pies, as well as side-by-side fibers (e.g., fibers having separate regions of polymers, wherein the substantially random interpolymer comprises at least a portion of the fiber's surface). Also included are the "islands in the sea" bicomponent fibers in which a cross section of the fiber has a main matrix of the first polymer component dispersed across which are extruded domains of a second polymer. On viewing a cross section of such a fiber, the main polymer matrix appears as a "sea" in which the domains of the second polymer component appear as islands.
The bicomponent fibers of the present invention can be prepared by coextruding the substantially random interpolymers in at least one portion of the fiber and the second polymer component in at least one other portion of the fiber. For all configurations in a sheath/core bicomponent fiber (i.e., one in which the sheath concentrically surrounds the core), the substantially random interpolymer can be in either the sheath or the core. Different substantially random interpolymers can also be used independently as the sheath and the core in the same fiber, and especially where the sheath component has a lower melting point than the core component. In the case of segmented pie configurations, one or more of the segments can comprise the substantially random interpolymer. In the case of an "island in the sea" configuration, either the islands or the matrix can comprise the substantially random interpolymer. The bicomponent fiber can be formed under melt blown, spunbond, continuous filament or staple fiber manufacturing conditions.
The shape of the shape memory fibers of the present invention is not limited. For example, typical fibers have a circular cross sectional shape, but sometimes fibers have different shapes, such as a trilobal shape, or a flat (i.e., "ribbon" like) shape. The fiber disclosed herein is not limited by the shape of the fiber.
Finishing operations can optionally be performed on the shape memory fibers of the present invention. For example, the fibers can be texturized by mechanically crimping or forming such as described in Textile Fibers, Dyes, Finishes, and Processes: A Concise Guide, by Howard L. Needles, Noyes Publications, 1986, pp. 17-20.
The polymer compositions used to make the shape memory fibers of the present invention or the fibers themselves may be modified by various cross-linking processes using curing methods at any stage of the fiber preparation including, but not limited to, before during, and after drawing at either elevated or ambient temperatures. Such cross-linking processes include, but are not limited to, peroxide-, silane-, sulfur-, radiation-, or azide-based cure systems. A full description of the various cross-linking technologies is described in copending U.S. patent application Ser. Nos. 08/921,641 and 08/921,642 both filed on Aug. 27, 1997, the entire contents of both of which are herein incorporated by reference.
Dual cure systems, which use a combination of heat, moisture cure, and radiation steps, may be effectively employed. Dual cure systems are disclosed and claimed in U.S. patent application Ser. No. 536,022, filed on Sep. 29, 1995, in the names of K. L. Walton and S. V. Karande, incorporated herein by reference. For instance, it may be desirable to employ peroxide crosslinking agents in conjunction with silane crosslinking agents, peroxide crosslinking agents in conjunction with radiation, sulfur-containing crosslinking agents in conjunction with silane crosslinking agents, etc.
The polymer compositions may also be modified by various cross-linking processes including, but not limited to the incorporation of a diene component as a termonomer in its preparation and subsequent cross linking by the aforementioned methods and further methods including vulcanization via the vinyl group using sulfur for example as the cross linking agent.
The shape memory fibers of the present invention may be surface functionalized by methods including, but not limited to sulfonation, chlorination using chemical treatments for permanent surfaces or incorporating a temporary coating using the various well known spin finishing processes.
Fabrics made from such novel shape memory fibers include both woven and nonwoven fabrics. Nonwoven fabrics can be made variously, including spunlaced (or hydrodynamically entangled) fabrics as disclosed in U.S. Pat. No. 3,485,706 (Evans) and U.S. Pat. No. 4,939,016 (Radwanski et al.), the disclosures of which are incorporated herein by reference; by carding and thermally bonding homofil or bicomponent staple fibers; by spunbonding homofil or bicomponent fibers in one continuous operation; or by melt blowing homofil or bicomponent fibers into fabric and subsequently calandering or thermally bonding the resultant web. Other structures made from such fibers are also included within the scope of the invention, including e.g., blends of these novel shape memory fibers with other fibers (e.g., poly(ethylene terephthalate) (PET) or cotton or wool or polyester).
Woven fabrics can also be made which comprise the shape memory fibers of the present invention. The various woven fabric manufacturing techniques are well known to those skilled in the art and the disclosure is not limited to any particular method. Woven fabrics are typically stronger and more heat resistant and are thus used typically in durable, non-disposable applications as for example in the woven blends with polyester and polyester cotton blends. The woven fabrics comprising the shape memory fibers of the present invention can be used in applications including but not limited to, upholstery, athletic apparel, carpet, fabrics, bandages such as, for example, elastic and non-elastic joint support bandages , ACE.TM. bandages and the like.
The novel shape memory fibers and fabrics disclosed herein can also be used in various fabricated articles as described in U.S. Pat. No. 2,957,512 (Wade), the disclosure of which is incorporated herein by reference. Attachment of the novel shape memory fibers and/or fabric to fibers, fabrics or other articles can be done with melt bonding or with adhesives. Gathered or shirred articles can be produced from the new fibers and/or fabrics and other components by pleating the other component (as described in U.S. Pat. No. '512) prior to attachment, prestretching the novel shape memory fiber component prior to attachment, or heat shrinking the novel shape memory fiber component after attachment.
The novel shape memory fibers described herein also can be used in a spunlaced (or hydrodynamically entangled) process to make novel structures. For example, U.S. Pat. No. 4,801,482 (Goggans), the disclosure of which is incorporated herein by reference, discloses a sheet which can now be made with the novel shape memory fibers/fabric described herein.
Composites that utilize very high molecular weight linear polyethylene or copolymer polyethylene also benefit from the novel shape memory fibers disclosed herein. For example, for the novel shape memory fibers that have a low melting point, such that in a blend of the novel shape memory fibers and very high molecular weight polyethylene fibers (e.g., Spectra.TM. fibers made by Allied Chemical) as described in U.S. Pat. No. 4,584,347 (Harpell et al.), the disclosure of which is incorporated herein by reference, the lower melting fibers bond the high molecular weight polyethylene fibers without melting the high molecular weight fibers, thus preserving the high strength and integrity of the high molecular weight fiber.
The fibers and fabrics can have additional materials which do not materially affect their properties. Such useful nonlimiting additive materials include pigments, antioxidants, stabilizers, surfactants (e.g., as disclosed in USP 4,486,552 (Niemann), U.S. Pat. No. 4,578,414 (Sawyer et al.) or U.S. Pat. No. 4,835,194 (Bright et al.), the complete disclosures of which are incorporated herein by reference).
Excellent teachings to processes for making ethylenic polymer foam structures and processing them are seen in C. P. Park, "Polyolefin Foam", Chapter 9, Handbook of Polymer Foams and Technology, edited by D. Klempner and K. C. Frisch, Hanser Publishers, Munich, Vienna, New York, Barcelona (1991), which is incorporated herein by reference.
Foam structures may take any physical configuration known in the art, such as sheet, plank, or bun stock. Other useful forms are expandable or foamable particles, moldable foam particles, or beads, and articles formed by expansion and/or coalescing and welding of those particles.
Foam structures may be made by a conventional extrusion foaming process. The structure is generally prepared by heating a polymer material to form a plasticized or melt polymer material, incorporating therein a blowing agent to form a foamable gel, and extruding the gel through a die to form the foam product. Prior to mixing with the blowing agent, the polymer material is heated to a temperature at or above its glass transition temperature or melting point.
Foam structures may also be formed in a coalesced strand form by extrusion of the ethylenic polymer material through a multi-orifice die. Apparatuses and methods for producing foam structures in coalesced strand form are seen in U.S. Pat. Nos. 3,573,152 and 4,824,720, both of which are incorporated herein by reference. Foam structures may also be formed by an accumulating extrusion process as seen in U.S. Pat. No. 4,323,528, which is incorporated by reference herein. Foam structures may also be formed into non-crosslinked foam beads suitable for molding into articles. This process is well taught in U.S. Pat. Nos. 4,379,859 and 4,464,484, which are incorporated herein by reference. In a derivative of this process, styrene monomer may be impregnated into the suspended pellets prior to impregnation with blowing agent to form a graft interpolymer with the ethylenic polymer material. The process of making the polyethylene/polystyrene interpolymer beads is described in U.S. Pat. No. 4,168,353, which is incorporated herein by reference.
The foam beads may then be molded by any means known in the art, such as charging the foam beads to the mold, compressing the mold to compress the beads, and heating the beads such as with steam to effect coalescing and welding of the beads to form the article. Some of the methods are taught in U.S. Pat. Nos. 3,504,068 and 3,953,558). Excellent teachings of the above processes and molding methods are seen in C. P. Park, supra, p. 191, pp. 197-198, and pp. 227-229, which are incorporated herein by reference.
There are many types of molding operations which can be used to form the structures and fabricated articles of the present invention, including, but not limited to, casting from solution, thermoforming and various injection molding processes (e.g., that described in Modern Plastics Encyclopedia/89, Mid October 1988 Issue, Volume 65, Number 11, pp. 264-268, "Introduction to Injection Molding" and on pp. 270-271, "Injection Molding Thermoplastics"), blow molding processes (e.g., that described in Modern Plastics Encyclopedia/89, Mid October 1988 Issue, Volume 65, Number 11, pp. 217-218, "Extrusion-Blow Molding"), compression molding, profile extrusion, sheet extrusion, film casting, coextrusion and multilayer extrusion, co-injection molding, lamination, film, spray coating, rotomolding and rotocasting.
However there remains a requirement for the such structures and fabricated articles prepared from polymer compositions which exhibit shape memory properties and have the ability to precisely tune both the glass transition process (location, amplitude and width of transition) in the vicinity of the ambient temperature (25.degree. C.), and the stiffness and modulus of the material in its final state. It would also be advantageous to prepare such structures and/or fabricated articles from polymer compositions which are easily processable and which exhibit shape memory properties without the requirement for cross linking. For many applications it would be highly desirable for the Tg of the polymer used to prepare the structure or fabricated article to be just above ambient temperature allowing for their use in applications requiring rigidity at room temperature but with access to the easily shaped rubbery stage by gentle heating to just above the Tg. Also the structure or fabricated article could then be returned to its original conformation by the same degree of gentle heating.
The present invention relates to structures and fabricated articles prepared from polymer compositions which comprise at least one substantially random interpolymer comprising polymer units derived from one or more .alpha.-olefin monomers with one or more vinyl or vinylidene aromatic monomers and/or a hindered aliphatic or cycloaliphatic vinyl or vinylidene monomers or blends therefrom. Unique to these novel materials are the shape memory properties of the polymer, coupled with their capacity to precisely tune both the glass transition process (location, amplitude and width of transition) in the vicinity of the ambient temperature range, and the stiffness and modulus of the material in its final state. Both these factors can be controlled by varying the relative amount of .alpha.-olefin(s) and vinyl or vinylidene aromatic and/or hindered aliphatic vinyl or vinylidene monomers in the final interpolymer or blend therefrom. Further increase in the Tg of the polymer composition used in the present invention can be introduced by variation of the type of component blended with the substantially random interpolymer including the presence of one or more tackifiers in the final formulation.
In a particularly preferred embodiment, the structure is a fiber prepared from the substantially random interpolymers or blend composition a plurality of which are used to form a styleable doll hair at room temperature. The hair is termed styleable because the shape memory properties of said fibers allow the doll hair to be placed or styled into a new conformation by heating above room temperature and the polymer Tg, such that the fibers are in their rubbery or moldable state, placing the hair in a new conformation or style, and cooling back below the room temperature and polymer Tg such that the fibers retain their original stiffness and modulus thus fixing the hair into the new style or configuration. The original style or configuration can be recovered simply by reheating the hair above room temperature and the polymer Tg.